1. Field of the Invention
This invention relates to an optical probe for detecting or irradiating evanescent light to be used for a near-field optical microscope. It also relates to a method of manufacturing such an optical probe.
2. Related Background Art
The recent development of the scanning tunneling microscope adapted for direct observation of the electron structure of surface atoms of a conductor has made it possible to visually observe an image of the real space of a specimen with an enhanced degree of resolution regardless of whether the specimen is monocrystalline or noncrystalline (G. Binnig et al., Phys. Rev. Lett. 49, 57 (1983)). Since then, research efforts have been paid on the scanning probe microscope (hereinafter referred to as SPM) particularly in an attempt to look into the micro-structures of various materials.
SPMs include scanning tunneling microscopes (STMs), atomic force microscopes (AFMs) and magnetic force microscope (MFMS) that are adapted to observe the surface structure of a specimen by detecting tunnel current, atomic force, magnetic force or light occurring when a probe having a micro-tip is brought very close to the specimen.
The scanning near-field optical microscope (hereinafter referred to as SNOM) has been developed from the STM. It can be used to examine the surface of a specimen by detecting evanescent light seeping out from the surface of the specimen being observed through the micro-aperture arranged at the micro-tip of the sharp probe of the microscope (Durig et al., J. Appl. Phys. 59, 3318 (1986)).
The photon STM (hereinafter referred to as PSTM) is a type of SNOM and adapted to examine the surface of a specimen by striking the specimen from the backside with a beam of light, making the beam totally reflected by the specimen, and then detecting evanescent light seeping out from the surface of the specimen by means of an optical probe (Reddick et al., Phys. Rev. B39, 767 (1989)).
Both the SNOM and the PSTM are designed to non-destructively observe the surface profile of the specimen including a micro pattern, if any, with an enhanced degree of positional resolution exceeding .lambda./2 that cannot be achieved by any conventional optical microscope, by utilizing evanescent light emitted through a very fine pin hole. Thus, they can be used to observe a wide variety of specimens including living bodies and cells of which observation was difficult and hence provide a broad scope of applicability. Since the resolution of the SNOM is defined by the diameter of the micro-tip of the optical probe, efforts have been paid to improve the optical probe. According to a proposed technique for the PSTM, for example, the micro-tip of the optical probe is sharpened by optimizing the conditions under which a chemical etching operation is conducted to etch an end surface of the optical fiber to be used as optical probe without arranging a micro-aperture at the tip of the optical probe in an attempt to improve the resolution. In the initial stages of development of the SNOM, a micro-aperture is formed by coating the crossing of cleaved planes of a transparent crystal with metal and then removing the metal from the crossing by pressing the metal-coated crossing against a very hard surface (European Patent EP0112402). More recently, a technique of preparing a micro-aperture by photolithography has been proposed. There is also a proposed technique of preparing an optical probe by forming a micro-aperture and an optical waveguide in an integrated fashion (U.S. Pat. No. 5,354,985).
However, if no micro-aperture is formed in the optical probe as described above by referring to the PSTM, the probe will detect not only evanescent light but also stray light scattered by the undulations of the surface of the specimen to consequently degrade the resolution of the microscope.
It is difficult to produce a plurality of fine optical probes in an integrated fashion by the known technique of etching an optical fiber. It is also difficult to arrange a plurality of optical probes in an array in order to improve the detection speed of the PSTM. Additionally, when an optical fiber is used for the SNOM or the PSTM, it has to be aligned with the optical axis of the light emitting device or the light receiving device which also makes it difficult to arrange optical probes in an array. If a plurality of optical fibers are used, then a plurality of drive means have to be used to drive the optical fibers to come closer to the surface of the specimen. Then, it is very difficult to densely arrange a large number of optical probes.
On the other hand, with the technique of forming a micro-aperture by means of photolithography, the surface of a substrate is processed by photolithograpy and etching. Then, the size of the minimal possible aperture that can be produced with such a technique will be 100 nm at most and it is practically impossible to produce a micro-aperture as small as 10 nm. Therefore, the resolution of the SNOM is inevitably limited by the above technical limitation. Additionally, this technique is time consuming and costly because it is complicated and involves complex processing steps. While it is theoretically possible to form a micro-aperture smaller than 100 nm by means of an electron beam processing apparatus or a focused ion beam processing apparatus, the use of such an apparatus entails an aligning process that has to be controlled in a complicated manner and a problem of dimensional irregularity of the produced optical probes. Additionally, optical probes have to be prepared on a one by one basis to reduce the yield and the throughput.